Conventionally, a device for forming an image by flying toner directly onto a recording medium has been well known as an image forming device for forming a visual image on a recording medium such as paper in response to an image signal (as disclosed by, for example, the Japanese Publication for Laid-Open Patent Application No. 6-286204/1994 (Tokukaihei 6-286204)). In an image forming device of this type, a toner carrier and a counter electrode are arranged vis-a-vis each other, and a control electrode having a plurality of passing pores (hereinafter referred to as gates) is provided therebetween. The image forming device controls the flight of the toner by controlling an electric field formed between the toner carrier and the control electrode by changing a voltage applied to the control electrode, and at the same time it causes the toner to reach a surface of a recording medium by using a strong electric field formed by the counter electrode, thereby forming an image on the recording medium.
The image forming device of this type forms an image directly on the recording medium, and hence it does not necessitate a photoconductive body and a visualizing body such as a dielectric drum which are used in conventional image forming devices. Therefore, a transfer operation for transferring an image from the visualizing body onto the recording medium is omitted, and hence the device causes no deterioration of the image, thereby resulting in improvement of the reliability of the device. Moreover, the arrangement of the device is simplified and the number of constituent parts is decreased, which results in reduction of the size of the device and lowering of the price of the device.
However, the conventional image forming device has a drawback in that discharge tends to occur between the counter electrode and the control electrode, or between the counter electrode and the toner carrier, which causes various problems to the device.
More specifically, a high voltage such as 1 kV to 2 kV is usually applied to the counter electrode. Since the counter electrode is provided close to the control electrode, or close to the toner carrier, normally with a distance therebetween of several millimeters, or sometimes with such a short distance as several hundred micrometers, discharge is easily caused therebetween by the high voltage. Therefore, even if the strength of the electric field between the counter electrode and the control electrode or the toner carrier is not greater than a dielectric strength, when paper dusts or unnecessary toner, or other impurities adhere to the counter electrode, the control electrode, or a toner layer on a surface of the toner carrier, the electric field concentratively affects the paper dusts or the like, easily causing insulation breakdown, resulting in discharge. As a result, a surface of the counter electrode, the toner carrier, or the control electrode is broken down.
The image forming device disclosed in the aforementioned publication is arranged so that when a sheet of paper ("substrate" in the publication) passes above the gates ("opening parts" in the publication) of the control electrode ("aperture electrode" in the publication), application of a high voltage to the counter electrode starts, and then, when the image formation based on image data for one page finishes, the control electrode is controlled so as to have an OFF potential, while the application of the high voltage to the counter electrode is suspended. Therefore, as compared with an arrangement wherein a high voltage is applied to the counter electrode without cessation, this image forming device is able to suppress the occurrence of discharge to some extent, but not completely.
The image forming device disclosed in the above publication is arranged so that even if a dimension of paper used in a direction orthogonal to a transporting direction is smaller than a dimension of the counter electrode in the same direction, a high voltage is applied throughout the counter electrode. Therefore, in the image forming device, in case of paper with specific dimensions, the counter electrode has a large exposed portion (a part uncovered with the paper), and it is impossible to avoid discharge occurring in the vicinity of this portion.
If discharge occurs, for example, on the surface of the counter electrode, a part of the surface of the counter electrode tends to be recessed, and moreover, a part surrounding the recessed part rises, thereby forming a protuberance. As a result, smooth transport of the recording medium becomes impossible. Besides, this causes such inconvenience as toner adheres to such recessed parts and soils a reverse surface of the recording medium.
Furthermore, if the discharge occurs on the surface of the toner carrier, the surface of the toner carrier tends to have recessed parts and protuberances, like in the case of the counter electrode. In this case, an adequate toner layer is not formed in areas where recessed parts and protuberances are formed, and this prevents toner from having desired properties, thereby making it difficult to control the toner flight. As a result, a good image forming operation cannot be achieved, thereby causing deterioration of image quality.
In addition, if the discharge occurs on the surface of the control electrode, as in the above case, the surface of the control electrode, that is, an insulating layer, is broken, and electrodes which are elements of the control electrode are exposed. If the toner adheres to the exposed control electrode, the potential of the control electrode changes from an appropriate value, due to electric charge that the toner has. As a result, desired control of the toner flight becomes difficult, and an adequate image forming operation cannot be achieved, thereby causing deterioration of image quality.
Furthermore, since protuberances formed on the control electrode, the toner carrier, or the counter electrode due to the discharge sharply extrude, these protuberances tend to cause secondary inconveniences such as another induction of discharge.
In the case where the discharge occurs on the surface of the toner carrier, control of the toner flight becomes impossible due to electric charge generated due to the discharge and shocks caused by the discharge, and even if a voltage which does not cause the toner to fly is applied to the control electrode, flight of toner to the control electrode or the counter electrode occurs.
Then, if another discharge occurs in this condition, the toner melts due to the discharge, and becomes fixed on the surface of the counter electrode, the toner carrier, or the control electrode. In such a case, the following inconveniences become conspicuous: smooth transport of a recording medium is hindered; and a reverse surface of the recording medium is soiled.
Furthermore, in the case where recessed parts or protuberances are formed, or the toner is fixed, on the surface of the control electrode, the toner carrier, or the counter electrode, adequate image formation cannot be conducted unless the member is replaced.
On the other hand, the discharge is also a factor causing electric noises. The noise generated due to the discharge is extremely strong and has a frequency ranging extremely widely. Therefore, there may occur an inconvenience that the control circuit catches the noise and erroneously operates, or the like. As a result, abnormal heat emission in a fixing section, or malfunction of a driving section, may be induced.
Besides, if discharge occurs, an electric current of a high voltage or a great electric current may possibly run directly into a power source that supplies desired voltages to members involved in the discharge, breaking electrodes of the power source and other control circuits.
Furthermore, when the conventional image forming device is connected with another apparatus, such as a host computer, the abnormal electric current runs through cables or the like thereto, possibly breaking the host computer.
Usually the above inconveniences are deemed avoidable by insulating the counter electrode, the toner carrier, or the control electrode. However, it should be noted that if a protective layer with a high resistance is formed on a surface of the counter electrode or the toner carrier, electric charge is generated on a surface of the protective layer due to friction of the protective layer with a recording medium or the toner, and the electric charge is accumulated in the protective layer, thereby, by taking the toner carrier as an example, making it impossible to confer appropriate properties on the toner and the toner layer. In the case of the counter electrode, a desirable electric field cannot be formed due to the potential of the electric charge. As a result, the control of the toner flight becomes difficult, and in a worst case, images cannot be formed.
On the other hand, in the case where to avoid the above problem, the resistance of the protective layer is lowered so as to neutralize the electric charge generated on the surface of the protective layer, the protection effect against discharge lowers. Moreover, in the case where the protective layer is formed on the surface of the toner carrier in the image forming device as represented by the above prior art, the protective layer has to be formed very thin, since the gap between the toner carrier and the control electrode is very narrow. Therefore, the protection effect of the protective layer against discharge becomes further lower. On top of that, since the toner carrier is always subjected to friction with toner and electric charge is accumulated therein, it is impossible to maintain the protective layer in a good condition due to its service life, thereby resulting in that control for forming a stable toner layer is difficult. For these reasons, under the present conditions, a protective layer cannot be provided on the toner carrier.